How do you experience self-doubt?
When I don't know who I am and what I am. I cannot discern what it is that defines the boundary of my ego from the world around me, that sets me apart.

What causes it?
Anxiety of not knowing and fully understanding myself.

What do you feel inside? What do you do or show on the outside?
Internally I feel anxiety. I may feel a need to withdraw, to ponder, reflect. What am I? How does this match up with what I know? I don't think much is shown on the outside more than my behavior to withdraw.

What is self-doubt to you?
Questioning yourself and what you know about yourself and the world.

What is "self" to you?
I need to come back to this when it's not 2:40 AM lol.

For me, it mostly manifests in indecision over the path of action I want to take. I am fairly assured of my identity, but I have a hard time making choices and acting on them. I get frustrated by that struggle, and then begin to question my ability to be successful.

What causes it?

Struggling to choose between paths, how to know which path will get me to the place I want to be. How to choose between two things that seem value-equal and simply different. How to know which is right or better, or which one will bring me the most long-term happiness.

What do you feel inside? What do you do or show on the outside?

I feel like I need more information from the outside, and I try to put everything in my life on hold as much as possible so I can research. It seems like I lose my footing completely without answers.

What is self-doubt to you?

A frustrating roadblock, but also an opportunity for learning.

What is "self" to you?

If only we knew, huh?

A nebulous collection of thoughts, ideas, feelings, emotions, actions, behaviors, body, spirit, mind, energy, and so on that comprise what I perceive to be "me".

Regarding the question of self, I honestly think it needs an exploration as its own topic because the boundaries between the self and the body for a 5 is something I think is often overlooked and misunderstood.